1. Field of the Invention
The present invention relates to a recording disc cartridge, and, more particularly, to a recording disc cartridge which has, for example, a flexible recording disc enclosed in a thin protective cartridge case.
2. Discussion of Related Art
In a magnetic recording disc cartridge of the above type, the flexible magnetic recording disc is rotatably supported in the cartridge case by a hub assembly, both upper and lower portions of which are situated in a drive shaft inspection window of the cartridge case so that the hub assembly is engaged with a drive unit of a magnetic recording/reproducing apparatus which is inserted through the drive shaft insertion window for rotating the magnetic recording disc.
More specifically, a supporting shaft or a spindle of the drive unit projecting at the center thereof is inserted in a central receiving hole of the hub assembly through the drive shaft insertion window and a drive shaft eccentrically situated with respect to the center of the drive unit is engaged with an engaging member formed in the hub assembly, so that rotation of the hub assembly or the magnetic recording disc is effected by rotation of the drive shaft of the drive unit with the axis of rotation being defined by the supporting shaft engaged in the central hole of the hub assembly.
In the magnetic recording disc cartridge of the above type, the hub assembly is required to be located in exactly the same position relative to the supporting shaft whenever the magnetic recording disc cartridge is mounted on the recording/reproducing apparatus otherwise, recording tracks traced by a magnetic recording head would be changed. In order to locate the hub assembly always in the exactly same position relative to the supporting shaft, the hub assembly must be made with a high degree of accuracy with respect to its geometric dimensions.
In order to solve this problem, U.S. Pat. No. 4,152,740 discloses a hub structure in which a driven surface for engagement with the drive shaft deviates from the radial center line of the receiving hole of the hub so that the drive shaft is engaged with the driven surface at such an angle that the hub is laterally moved to bring the wall of the receiving hole into contact with the supporting shaft. By virtue of this construction, contact is ensured at two unique positions, i.e., between the drive shaft and the driven surface and between the supporting shaft and the wall of the receiving hole, so that the hub is always located in the exactly the same position relative to the supporting shaft.
However, in the case of the hub structure described above, if the force exerted on the drive shaft is changed due to change of frictional force between a recording head and a magnetic recording disc, the contact point between the drive shaft and the driven surface is accordingly changed, resulting in displacement of the contact point between the supporting shaft and the wall of the receiving hole, and thus the hub will not be located in the same position relative to the supporting shaft.